1. Field of the Invention
The present invention relates to a solvent-free aqueous resin dispersion, a process for preparing such a dispersion, and a method of using such a dispersion.
2. Description of Related Art
In electrodeposition coating, the electrodeposition coating materials preferably employed are those whose binders comprise cationic, amine-modified epoxy resins as water-soluble base resin components. These resins can be crosslinked by being condensed and/or admixed with polyisocyanates which are blocked with alcohol groups and/or amino groups. The polyisocyanates at room temperature possess urethane and/or urea capping groups and, at elevated temperatures, give off the capping components and lead to crosslinking with the free hydroxyl groups and/or amino groups of the base resin.
For reasons of viscosity or reactivity, it is necessary to employ solvents in the preparation of the individual binder components. Thus it is preferred in the synthesis of the base resins to use protic solvent such as alcohols and glycols, and aprotic solvents such as ketones, esters or aromatic compounds, and to use aprotic solvents in the synthesis of the crosslinking component.
However, it is known that, as the solvent content in cathodic electrodeposition baths increases, there is a worsening in the throwing power and coating of cavities, instances of overcoating occur, and there is increased passage of solvents into the waste water and/or waste air. For this reason the solvents must be removed from the binders before and/or after dispersion in water, for example by distillation or ultrafiltration. It is inherent in this process that some of the solvent remains in the dispersions, and, especially when carrying out so-called stripping from the aqueous phase and ultrafiltration, relatively large quantities are obtained of a solvent/water mixture which has to be disposed of or reprocessed.